It is common knowledge that lead and its compounds are highly toxic. Both traditional and novel methods for producing lead are carried out at high temperatures in the order of 1200.degree. to 1300.degree. C. At these temperatures lead and its compounds possess a high volatility, and, as a result, the pass in considerable quantities to gas phase, thereby polluting the environment with high-toxicity lead and sulphurous gas. Not a single of the known methods for lead recovery is able to ensure adequate environmental control at operator's positions and in areas adjoining to such lead-recovering enterprises. As a result, the lead concentration in atmosphere exceeds by scores of times the limit permissible levels. Apart from it, the know methods impose restrictions upon the contents of zinc and copper admixtures in concentrates, and these restrictions make it impossible to process concentrates with lead contents of above 45%. On the other hand, use of high temperatures in production of low-melting lead is economically unjustified and, therefore, the most promising process for lead recovery is the low-temperature process described in the "Tsvetnye Metally"/ Non-Ferrous Metals J./, No. 5, 1990, Metallurgia Publishers, Moscow, pp. 34-26, FIG. 1.
The above-cited process makes provision for feeding lead-containing pulverized sulphide concentrated and molten alkali to a reaction zone in which a temperature in the order of 600.degree.-700.degree. C. is maintained. To provided adequate fluidity of the melt within said temperature range, caustic soda is added in excess.
The lead reduction process is the reaction zone preceeds in accordance with the following formula: EQU 4 PbS+8 NaOH=4 Pb .BECAUSE.+3 Na.sub.2 S+Na.sub.2 SO.sub.4 +4 H.sub.2 O.uparw. (1)
The above process gives leads yields of 96-98%.
The crude lead thus-obtained is withdrawn from the process for subsequent refining. The sulphur associated with lead reacts with caustic soda to form salts Na.sub.2 S and Na.sub.2 SO.sub.4 which, in addition to excess NaOH, represent the constituents of the melt. The latter also comprises sulphides of other heavy metals, such as, e.g. ZnS, CU.sub.2 S and barren rock. The metal is subjected to hydrometallurgical treatment for the purpose of regeneration of alkali and for removal of zinc and copper in the form of intermediate products. To reduce sodium sulphate Na.sub.2 SO.sub.4 to sodium sulphide Na.sub.2 S, coal or converted natural gas are introduced into the melt. The process is conducted at 800.degree.-850.degree. C. The melt thus-treated containing Na.sub.2 S and excess caustic soda is subjected to leaching with an aqueous solution to obtain a slurry or pulp. Zinc oxide in the form of roasted cinder is added to the slurry and, as a result, of its chemical reaction with sodium sulphide, there are produced caustic soda in the form of a solution and zinc sulphide as a solid particulate matter, i.e. a slurry which is subjected to filtering. As a result of filtering, caustic soda solution and cake containing zinc sulphide and barren rock from the feed lead-containing concentrate are obtained. The caustic soda solution is directed to dewatering to eventually obtain a caustic soda melt fed to the reaction zone, while the cake is washed with water. Thereupon, the cake is dried and fired at a temperature of 950.degree.-1000.degree. C. to produce zinc oxide in the form of zinc cinder and sulphurous gas SO.sub.2 used for the production of sulphuric acid.
Since the above-described lead recovery process is carried out within the temperature range of 600.degree. to 700.degree. C., considerably less lead and lead compounds pass to gas phase, thus reducing the volume of process gases by a factor of 20 to 30. As a result, the cleaning of process gases is much simplified and it becomes easier to achieve in them the prescribed permissible lead concentration. All these factors improve sanitary and hygienic work conditions, reduce gas discharges and air pollution.
It is regeneration of caustic soda from the melt that constitutes the decisive step of the above-described process. This regeneration step is not concerned with toxic lead and, on the whole, it improves the environmental aspect of the lead production. The lead produced by the above-described process is purer than that produced by any traditional method, and it contains only noble metals, bismuth and a minor amount (.ltoreq.0.1%) of copper. With the flow-sheet adopted for carrying out the above-described process, more than 98% of noble metals and bismuth are recovered from feed concentrate, while arsenic, tin, antimony pass to the melt. The absence of these impurities, particularly copper, in lead considerably simplifies the subsequent refining and makes it possible to increase the percentage of recovery of lead, noble metals and bismuth at this process step.
The above-described process makes it possible to recover lead from feed concentrates containing high percentages of zinc admixture, e.g. up to 11%, and copper, e.g. up to 20%, as well as high percentages of lead, e.g. up to 70-80%.
One of the specific features of the above-described process resides in the need to add coal or converted natural gas to the melt to reduce sodium sulphate to sodium sulphide.
This reduction process is carried out in a separate furnace unit at elevated temperatures in the order of 800.degree.-850.degree. C. to cause evaporation of alkali. The process is accompanied by side reactions in the melt: EQU C+O.sub.2 =CO.sub.2 EQU 2NaOH+CO.sub.2 =Na.sub.2 CO.sub.3 +H.sub.2 O
leading to conversion of free caustic soda contained in the melt to sodium carbonate. This side reaction considerably complicates regeneration of caustic soda from the melt and necessirates an additional soda caustification operation.
To carry out NaOH regeneration, the art-known process comprises the operation of treating sodium sulphide solution with zinc oxide in the form of roasted zinc cinder which is a valuable product. As a result of a reaction between sodium sulphide and zinc oxide, the latter passes to zinc sulphide. Since roasted zinc cinder is a valuable product, it must be regenerated. For this purpose, a roasting operation is carried out at high temperatures of 950.degree. to 1000.degree. C.
The oxidation process is accompanied by liberation of sulphurous gas SO.sub.2, whereby the environmental aspects are impaired.
Also known in the prior art is another process for recovering lead from lead-containing raw materials described by M. P. Smirnov and L. N. Kudriashova in the article "Alkaline Lead Melting Process" in the "Tsvetnye Metally" /Non-Ferrous Metals Journal/, Moscow, Metallurgizdat Publishers, 1958, No. 9, pp. 14-23.
The latter process is carried out in the following manner into a reaction zone molten caustic soda and a lead-containing feed, such as, e.g. plumbiferous sulphide concentrates in the form of a powder, are fed. A temperature of 600.degree. to 700.degree. C. is maintained in the reaction zone. To boost up the melting process, the melt formed in the reaction zone is subjected to mechanical stirring. Melting is conducted in air atmosphere. The bulk of lead metal is at once extracted from the concentrate, and this crude lead is withdrawn from the process for subsequent refining. Sulphur and other components of the concentrate pass to the melt which is subjected to hydrometallurgical treatment for the purpose of regenerating caustic soda and for producing zinc and copper in the form of an intermediate product removed from the process.
In accordance with the reaction (I), the sulphur associated with the lead reacts with caustic soda to form salts Na.sub.2 S and Na.sub.2 SO.sub.4 which constitute the melt components. Regeneration of caustic soda from the melt is effected in three steps:
Ist STEP--removal of free caustic soda from the melt by conducting a first leaching operation, followed by filtering to obtain a solid residue to be subjected to a second leaching operation;
IInd STEP--sulphidization of the solution obtained by said second leaching operation and containing mainly sodium sulphate by using barium sulphide;
IIIrd STEP--caustification of the solution obtained by sulphidization treatment and containing mainly sodium sulphide by using copper oxide.
Upon completion of the 1st regeneration step, a strong Na.sub.2 OH solution is obtained. However, sodium sulphide Na.sub.2 S from the melt partially passes to the strong NaOH solution, and partially to a solid residue mainly containing sodium sulphate Na.sub.2 SO.sub.4.
The 2nd step of caustic soda regeneration proceeds as follows: EQU Na.sub.2 SO.sub.4 +BaS=Na.sub.2 S+BaSO.sub.4 .dwnarw. (2)
The precipitated BaSO.sub.4 is subjected to treatment, in the course of which it is thermally reduced with carbon at the temperatures of above 1000.degree. C. to obtain gaseous carbon dioxide CO.sub.2 and barium sulphide BaS returned back to the IInd alkali regeneration step.
The IIIrd step of caustic soda regeneration proceeds as follows: EQU Na.sub.2 S+CuO+H.sub.2 O=2NaOH+CuS.dwnarw. (3)
The precipitated CuS is subjected to thermal oxidation with oxygen of the air at a temperature of above 1000.degree. C. As a result of thermal oxidation, sulphurous anhydride SO.sub.2 and copper oxide CuO are produced, CuO being returned to the IIIrd alkali regeneration step.
The concentrated caustic soda solution obtained in the Ist alkali regeneration step and contaminated with sodium sulphide Na.sub.2 S is subjected to caustification operation with copper oxide CuO, this caustification treatment being carried out similarly to the IIIrd alkali regeneration step.
Further on, the caustic soda solution obtained by the IIIrd alkali regeneration step is combined with the caustic soda solution obtained by the Ist regeneration step and subjected to caustification treatment. The combined caustic soda solution is subjected dewatering and, as a result, a caustic soda melt is obtained which is returned to the reaction zone.
Amongst drawbacks of the latter process for lead recovery from plumbiferous concentrates, one should mention considerable expenses for carrying out alkali regeneration because of the need to organize additional production units for regeneration of the sulphidizing agent (i.e. barium sulphide) and causticizing agent (i.e. copper oxide).
The operations for regenerating barium sulphide and copper oxide are associated with the need to use high temperatures and are accompanied of environmentally harmful discharges of gaseous media (such as CO, SO.sub.2, etc.).
Since a portion of sodium sulphide passes to the strong caustic soda solution, while its remaining portion --together with sodium sulphate--passes to the weak sodium sulphate solution obtained by the 2nd leaching operation, it becomes necessary to causticize sodium sulphide in both branches of the process flow-sheet, whereby it becomes much complicated.
Besides, operation with solutions containing sodium sulphide calls for considerable expenditure for environment protective measures to prevent discharges of hydrogen sulphide to the environment.